1. Field of the Invention
The present invention relates in general to vaporizers to convert a liquid feed to a vapor feed for chemical vapor deposition apparatus, and relates in particular to a vaporizer section suitable for a vapor feed material for depositing a highly dielectric or ferroelectric thin film, such as barium or strontium titanate.
2. Description of the Related Art
In recent years, there has been a quantum jump in circuit density of integrated circuit devices produced by the semiconductor industries, and intense development activities are underway in anticipation of giga-bit order DRAMs replacing the prevailing mega-bit order DRAMs of today. Dielectric thin film materials used to make high capacitance devices necessary for producing DRAMs have, in the past, included silicon oxide or silicon nitride films of dielectric constant less than 10, and tantalum pentaoxide (Ta.sub.2 O.sub.5) films of dielectric constant less than 20; However, newer materials such as barium titanate (BaTiO.sub.3), strontium titanate (SrTiO.sub.3) or barium-strontium titanate (BST), having dielectric contestants of about 300, appear to be more promising. Promising also are even higher dielectric materials such as lead-zinc-titanate (PZT), lead-lithium-zinc-titanate (PLZT) and Y1.
Of the various methods of making such thin films, prospects are particularly bright for the chemical vapor deposition (CVD) process, and in this case, it is necessary that a gaseous feed must ultimately be supplied in a steady gas stream to a substrate disposed in the deposition chamber. The gaseous feed is produced by heating a liquid mixture of liquefied materials such as Ba(DPM).sub.2 or Sr(DPM).sub.2 (which is solid at normal temperature) and some organic solvent such as THF (Tetrahydrofuran) for stabilization of the vaporization characteristics. Some known examples of vaporizer sections include, for example, those that atomize the liquid feed through spray nozzles or ultrasonic transducers, and deliver the atomized mist to a high temperature zone to convert the mist to a gaseous feed.
However, it is extremely difficult to produce thermodynamically stable vapors of such highly dielectric and ferroelectric materials mentioned above. This is because, for these materials, 1 the vaporization and decomposition temperatures are close; 2 the vaporization temperature for the liquid feed material is different from that for the organic solvent; 3 the vapor pressures are very low; and 4 the materials are vulnerable to react with a small amount of oxygen, vapor water, etc.
For example, in a liquid feed obtained by dissolving Ba(DPM).sub.2 or Sr(DPM).sub.2 in THF, the solvent exists as a liquid in region (a) in FIG. 34, and the feed material exists as a liquid or solid in region (a+c). In region (b), the feed is totally a vapor. Therefore, when the liquid feed in region (a) is heated to be converted into a vapor feed and passes through the region (c), only the solvent vaporizes, leaving the solute components in the liquid feed to precipitate out. This results in blocking of the gas passages or quality degradation due to changes in composition. For this reason, it is considered important to heat the liquid feed to its high temperature vaporization region as rapidly as possible.
Furthermore, depending on the film material or film deposition conditions, it is sometimes necessary to supply the feed vapor at a minute rate to the deposition chamber. If the process of vaporization is not conducted smoothly and the gaseous feed delivery to the deposition chamber becomes unstable, deposition process will be seriously affected. Therefore, it is important to be able to control the vaporization of the gaseous feed down to very low flow rates.
In the conventional technologies for atomizing the feed liquid based on spray nozzles, it is difficult to control atomization at low flow rates of liquid feed, because of the high pressures used to atomize the liquid. In the ultrasonic atomization techniques, it is difficult to find transducer materials which can withstand the high temperatures used for vaporization. Additionally, it is desirable to carry out the liquid-to-vapor conversion process physically near the deposition chamber so as to minimize the distance of transport. However, the above-mentioned apparatus is usually designed to atomize first and then to vaporize so that it is difficult to make the apparatus small. Also, both techniques require fairly large facilities for atomization and spray purposes, and it is unavoidable that stagnant regions of liquid feed are created within the apparatus, and degradation of the liquid feed and difficulty in controlling the flow rates of gaseous feed are experienced in the current technologies.